Nonvolatile configuration cells and cell arrays

ABSTRACT

A memory cell (400) used to store data in an integrated circuit. The memory cell (400) is static, nonvolatile, and programmable. The layout of the memory cell is compact. A logic high output from the memory cell (400) is about VDD and a logic low output is about VSS. The memory cell (400) of the present invention includes a programmable memory element (515). In one embodiment, the programmable memory element (515) is coupled between supply voltage (510) and an output node (405). A pull-down device (525) is coupled between another supply voltage (505) and the output node (405). The memory cell (400) may be used to store she configuration information for a programmable logic device (121).

This is a division of application Ser. No. 08/710,398 filed Sep. 16,1996, which claims benefit of a provisional application Ser. No.60/013,435, filed Mar. 14, 1996, both of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of integrated circuit memorytechnology. More specifically, the present invention provides a staticnonvolatile memory cell for storing data.

Memory cells are used in the implementation of many types of electronicdevices and integrated circuits. These devices include microprocessors,static random access memories (SRAMs), erasable-programmable read onlymemories (EPROMs), electrically erasable programmable read only memories(EEPROMs), Flash EEPROM memories, programmable logic devices (PLDs),field programmable gate arrays (FPGAs), application specific integratedcircuits (ASICs), among others. Memory cells are used to store the dataand other information for these and other integrated circuits.

As integrated circuit technology and semiconductor processing continueto advance, there is a need for greater densities and functionality inintegrated circuits, which are often determined in a large part by thesize of the memory cells. Further, it is desirable that the memory cellshave improved operating characteristics, such as lower powerconsumption, nonvolatility, greater device longevity, improved dataretention, better transient performance, superior voltage and currentattributes, and improvements in other similar attributes.

Furthermore, improved memory cells are especially needed for particularapplications, such as PLD integrated circuits. PLDs are well known tothose in the electronic art. Such programmable logic devices arecommonly referred as PALs (Programmable Array Logic), PLAs (ProgrammableLogic Arrays), FPLAs, PLDs, EPLDs (Erasable Programmable Logic Devices),EEPLDs (Electrically Erasable Programmable Logic Devices), LCAs (LogicCell Arrays), FPGAs (Field Programmable Gate Arrays), and the like. Suchdevices are used in a wide array of applications where it is desirableto program standard, off-the-shelf devices for a specific application.Such devices include, for example, the well-known, Classic™, and MAX®5000, MAX® 7000, and FLEX® 8000 EPLDs made by Altera Corp.

PLDs are generally known in which many logic array blocks (LABs) areprovided in a two-dimensional array. LABs contain a number of individualprogrammable logic elements (LEs) which provide relatively elementarylogic functions such as NAND, NOR, and exclusive OR. Further, PLDs havean array of intersecting signal conductors for programmably selectingand conducting logic signals to, from, and between the LABs and LEs. Theconfiguration of the LABs, LEs, and interconnections between theselogical elements is stored in memory cells. Memory cells may be used toprogrammably control the composition, configuration, and arrangements oflogic array blocks (LABs) and logic elements (LEs) and also theinterconnections between these logic array blocks and logic elements.

Resulting from the continued scaling and shrinking of semiconductordevice geometries which are used to form integrated circuits (also knownas "chips"), integrated circuits have progressively become smaller anddenser. For programmable logic, it becomes possible to put greaternumbers of programmable logic elements onto one integrated circuit.Furthermore, as the number of elements increases, it becomesincreasingly important to improve the techniques and architectures usedfor interconnecting the elements and routing signals between the logicblocks. Also as PLDs increase in size and complexity, greater numbers ofmemory cells are required on to hold the configuration information ofthe logical elements.

While such devices have met with substantial success, such devices alsomeet with certain limitations, especially in situations in which theprovision of more complex logic modules and additional or alternativetypes of interconnections between the logic modules would have benefitssufficient to justify the additional circuitry and programmingcomplexity. There is also a continuing demand for logic devices withlarger capacity. This produces a need to implement logic functions moreefficiently and to make better use of the portion of the device which isdevoted to interconnecting individual logic modules. Moreover, there isa need to more efficiently and effectively store the configurationinformation of PLDs. The memory technology used to store theconfiguration information of the PLD should be compact, power efficient,programmable and nonvolatile, require little additional programmingcircuitry overhead, and generally provide enhancements to thePerformance and features of PLD logic modules and interconnections.

As can be seen, an improved memory cell is needed, especially animproved memory cell for storing the configuration information for thelogic elements and interconnections of a programmable logic device.

SUMMARY OF THE INVENTION

The present invention provides a static, nonvolatile, and programmablememory cell for storing data in an integrated circuit. The memory cellof the present invention includes a programmable memory element. Theprogrammable memory element is coupled between a voltage source, such asVDD or VSS, and an output node. The programmable memory element may befabricated using many different memory technologies, including antifuse,EPROM, EEPROM, and Flash EEPROM, to name a few. In one embodiment, theprogrammable memory element is coupled between VDD and the output node.A pull-down device is coupled between VSS and the output node. Inanother embodiment, the programmable memory element is coupled betweenVSS and the output node. A pull-us device is coupled between VDD and theoutput node. The pull-down or pull-up device may be a resistor, amongother devices, which may be formed using diffusion polysilicide,polysilicon, thin-film transistor, or other structure or material.

Operation of the invention when the programmable memory element iscoupled between VDD and the output node is as follows. When theprogrammable memory element is erased, the memory cell stores andoutputs a logic high at the output node. When the programmable memoryelement is programmed, the memory cell stores and outputs a logic low atthe output node. A logic high output from the memory cell is about VDDand a logic low output is about VSS. More specifically, when theprogrammable memory element is programmed, a first pull-down current, orstandby pull-down current, to VSS through the pull-down device will pullthe output node to about VSS. Except for leakage currents, the firstpull-down current may be about zero microamps. In this state, the memorycell consumes no static power.

When the programmable memory element is erased, the output node will becoupled through the programmable memory element to VDD. A secondpull-down current to VSS through the pull-down device will be drainedthrough the programmable memory element to VSS. The output node will beat about VDD. The second pull-down current may be much less than onemicroamp. Operation of the invention when the programmable memoryelement is coupled between the output node and VSS would be analogous tothis discussion.

In an embodiment of the present invention, the resistance of thepull-down device is substantially less than the off resistance of theprogrammable memory element when the programmable memory element isprogrammed. The resistance of the pull-down device is substantially morethan the on resistance of the programmable memory element when theprogrammable memory element is erased.

The memory cell of the present invention is extremely compact. Thelayout of the memory cell is compact. The memory cell has extremely lowpower consumption. A plurality of memory cells of the present inventionhas relatively low power consumption. The memory cell may be used tostore the configuration information for a programmable logic device.

More specifically, the memory cell of the present invention includes: anoutput node, for providing approximately full-rail output voltages; anda pull-down device, coupled between a first voltage source at a firstvoltage level and the output node. The pull-down device provides a firstpull-down current when the memory cell stores a logic low and a secondpull-down current when the memory cell stores a logic high. The memorycell further includes a nonvolatile programmable memory element, whichis coupled between the output node and a second voltage source at asecond voltage level. The second voltage level is above the firstvoltage level. Further, the nonvolatile programmable memory elementstores data and retains this data, even when power is removed from theintegrated circuit. The first pull-down current pulls the output node toabout the first voltage level when the nonvolatile programmable memoryelement is programmed. And, the second pull-down current is drainedthrough the nonvolatile programmable memory element from the secondvoltage source so that the output node is about the second voltage levelwhen the nonvolatile programmable memory element is erased.

In a further embodiment, the present invention is a programmable logicdevice including: a first plurality of conductors extending in a firstdirection for conducting logic signals; and a second plurality ofconductors in a second direction, transverse to said first direction,for conducting logic signals; and a plurality of programmableintersections for programmably coupling the first plurality ofconductors to the second plurality of conductors. A programmableintersection includes: an output node for providing approximatelyfull-rail output voltages; a pull-down resistor, coupled between a firstvoltage source at a first voltage level and the output node; and anonvolatile memory element, coupled between the output node and a secondvoltage source at a second voltage level, where this second voltagelevel is above the first voltage level. Further, the nonvolatile memoryelement stores data and retains this data, even when power is removedfrom the programmable logic device.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a digital system incorporating aprogrammable logic device integrated circuit;

FIG. 2A is a block diagram showing an architecture for a programmablelogic device;

FIG. 2B is a block diagram showing an architecture for a programmablelogic device;

FIG. 2C is a block diagram showing an architecture for a programmablelogic device;

FIG. 2D is a block diagram showing a segmented programmable interconnectarchitecture for a programmable logic device;

FIG. 2E shows a description of the meanings of the symbols used in FIG.2D;

FIG. 3 is a simplified block diagram of a logic array block (LAB) of aprogrammable logic device;

FIG. 4 is a diagram of the memory cell of the present invention used inthe programmable interconnect of a programmable logic device;

FIG. 5A is a block diagram of a memory cell of the present invention;

FIG. 5B is a block diagram of a memory cell of the present inventionincorporating a thin film transistor;

FIG. 5C is a block diagram of an alternative embodiment of the memorycell of the present invention;

FIG. 6 shows a memory cell of the present invention implemented usingEEPROM cells;

FIG. 7 shows a memory cell of the present invention implemented usingtwo-transistor double-polysilicon Flash EEPROM memory cells; and

FIG. 8 shows another embodiment of a memory cell of the presentinvention implemented using Flash EEPROM memory cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a digital system within which thepresent invention may be embodied. In the particular embodiment of FIG.1, a processing unit 101 is coupled to a memory 105 and an I/O 111 andincorporates a programmable logic device (PLD) 121. PLD 121 may bespecially coupled to memory 105 through connection 131 and to I/O 111through connection 135. The system may be a programmed digital computersystem, digital signal processing system, specialized digital switchingnetwork, or other processing system. Moreover, the system may be ageneral purpose computer, a special purpose computer (such astelecommunications equipment) optimized for an application-specific tasksuch as programming PLD 121, or a combination of a general purposecomputer and auxiliary special purpose hardware.

Processing unit 101 may direct data to an appropriate system componentfor processing or storage, execute a program stored in memory 105 orinput using I/O 111, or other similar function. Processing unit 101 maybe a central processing unit (CPU), microprocessor, floating pointcoprocessor, graphics coprocessor, hardware controller, microcontroller,programmable logic device programmed for use as a controller, or otherprocessing unit. Furthermore, in many embodiments, there is often noneed for a CPU. For example, instead of a CPU, one or more PLDs 121 maycontrol the logical operations of the system.

In some embodiments, processing unit 101 may even be a computer system.In one embodiment, source code may be stored in memory 105, compiledinto machine language, and executed by processing unit 101. Processingunit 101 need not contain a CPU and in one embodiment, instructions maybe executed by one or more PLDs 121. Instead of storing source code inmemory 105, only the machine language representation of the source code,without the source code, nay be stored in memory 105 for execution byprocessing unit 101. Memory 105 may be a random access memory (RAM),read only memory (ROM), fixed or flexible disk media, PC Card flash diskmemory, tape, or any other storage retrieval means, or any combinationof these storage retrieval means.

Processing unit 101 uses I/O 111 to provide an input and output path foruser interaction. For example, a user may input logical functions to beprogrammed into programmable logic device 121. I/O 111 may be akeyboard, mouse, track ball, digitizing tablet, text or araphicaldisplay, touch screen, pen tablet, printer, or other input or outputmeans, or any combination of these means. In one embodiment, I/O 111includes a printer used for printing a hard copy of any processing unit101 output. In particular, using I/O 111, a user may print a copy of adocument prepared using a word processing program executed usingprocessing unit 101. In other cases, a user may print out a copy of thesource code or a listing of the logical functions contained within PLD121.

PLD 121 may serve many different purposes within the system in FIG. 1.PLD 121 may be a logical building block of processing unit 101,supporting its internal and external operations. PLD 121 is programmedto implement the logical functions necessary to carry on its particularrole in system operation.

FIG. 2A is a simplified block diagram of an overall internalarchitecture and organization of PLD 121 of FIG. 1. Other architecturesare shown in FIGS. 2B-D. Many details of PLD architecture, organization,and circuit design are not necessary for an understanding of the presentinvention and such details are not shown in FIGS. 2A-D.

FIG. 2A shows a six-by-six two-dimensional array of thirty-six logicarray blocks (LABs) 200. LAB 200 is a physically grouped set of logicalresources that is configured or programmed to perform logical functions.The internal architecture of a LAD will be described In more detailbelow in connection with FIG. 3. PLDs may contain any arbitrary numberof LABs, more or less than the PLD 121 shown in FIG. 2A. Generally, inthe future, as technology advances and improves, programmable logicdevices with even greater numbers of logic array blocks will undoubtedlybe created. Furthermore, LABs 200 need not be organized in a squarematrix; for example, the array may be organized in a five-by-seven or atwenty-by-seventy matrix of LABs.

LAB 200 has inputs and outputs (not shown) which may or may not beprogrammably connected to a global interconnect structure, comprising anarray of global horizontal interconnects (GHs) 210 and global verticalinterconnects (GVs) 220. Although shown as single lines in FIG. 2A, eachGH 210 and GV 220 line may represent a plurality of signal conductors.The inputs and outputs of LAB 200 are programmably connectable to anadjacent GH 210 and an adjacent GV 220. Utilizing GH 210 and GV 220interconnects, multiple LABs 200 may be connected and combined toimplement larger, more complex logic functions than can be realizedusing a single LAB 200.

In one embodiment, GH 210 and GV 220 conductors may or may not beprogrammably connectable at intersections 225 of these conductors.Moreover, GH 210 and GV 220 conductors may make multiple connections toother GH 210 and GV 220 conductors. Various GH 210 and GV 220 conductorsmay be programmably connected together to create a signal path from aLAB 200 at one location on PLD 121 to another LAB 200 at anotherlocation on PLD 121. Furthermore, an output signal from one LAB 200 canbe directed into the inputs of one or more LABs 200. Also, using theglobal interconnect, signals from a LAB 200 can be fed back into thesame LAB 200. In other embodiments or the present invention, onlyselected GH 210 conductors are programmably connectable to a selectionof GV 220 conductors. Furthermore, in still further embodiments, GH 210and GV 220 conductors may be specifically used for passing signal in aspecific direction, such as input or output, but not both.

The PLD architecture in FIG. 2A further shows at the peripheries of thechip, input-output drivers 230. Input-output drivers 230 are forinterfacing the PLD to external, off-chip circuitry. FIG. 2A showsthirty-two input-output drivers 230; however, a PLD may contain anynumber of input-output drivers, more or less than the number depicted.Each input-output driver 230 is configurable for use as an input driver,output driver, or bidirectional driver.

FIG. 2B shows a simplified block diagram of an alternative PLDarchitecture. In this embodiment, a plurality of LABs 200 may beprogrammably coupled using GHs 210 and GV 220. This architecture mayalso have input-output drivers 230, but are not shown in order tosimplify the diagram. The input-output driver functions may also beincorporated within LABs 200, or some LABs 200. The memory cell of thepresent invention may be used to store the configuration of thecomponents of this PLD architecture.

FIG. 2C shows a simplified block diagram of another PLD architecture. Inthis embodiment, a plurality of LABs 200 may be programmably coupledusing GV 220. This architecture may also have input-output drivers 230,but are not shown in order to simplify the diagram. The memory cell ofthe present invention may be used to store the configuration of thecomponents of this PLD architecture.

FIG. 2D shows a block diagram of another PLD architecture. In thisembodiment, a plurality of LABs 200 may be programmably coupled using a"segmented" programmable interconnect. FIG. 2E shows a description ofthe meanings of the interconnect symbols used in FIG. 2D. A plurality ofinput-output drivers 230 surround the LABs 200. The architecture in FIG.2D has segmented programmable interconnect, where signals and LABs 200are coupled using programmable interconnect segments, which do notnecessarily run the entire length of a row or column of LABs 200. Thememory cell of the present invention may be used to store theconfiguration of the components of this PLD architecture.

FIG. 3 shows a simplified block diagram of LAB 200 of FIGS. 2A-D. LAB200 is comprised of a varying number of logic elements (LEs) 300,sometimes referred to as "logic cells," and a local (or internal;interconnect structure 310. LAB 200 has eight LEs 300, but LAB 200 mayhave any number of LEs, more or less than eight. In a further embodimentof the present invention, LAB 200 has two "banks" of eight LEs for atotal of sixteen LEs, where each bank has separate inputs, outputs,control signals, and carry chains.

A general overview of LE 300 is presented here, sufficient to provide abasic understanding of the present invention. LE 300 is the smallestlogical building block of a PLD. Signals external to the LAB, such asfrom GHs 210 and GVs 220, are programmably connected to LE 300 throughlocal interconnect structure 310, although LE 300 may be implemented inmany architectures other than those shown in FIGS. 1-3. In oneembodiment, LE 300 of the present invention incorporates a functiongenerator that is configurable to provide a logical function of a numberof variables, such a four-variable Boolean operation. As well asccmbinatorial functions, LE 300 also provides support for sequential andregistered functions using, for example, D flip-flops.

LE 300 provides combinatorial and registered outputs that areconnectable to the GHs 210 and GVs 220, outside LAB 200. Furthermore,the outputs from LE 300 may be internally fed back into localinterconnect structure 310; through local interconnect structure 310, anoutput from one LE 300 may be programmably connected to the inputs ofother LEs 300, without using the global interconnect structure's GHs 210and GVs 220. Local interconnect structure 310 allows short-distanceinterconnection of LEs, without utilizing the limited global resources,GHs 210 and GVs 220. Through local interconnect structure 310 and localfeedback, LEs 300 are programmably connectable to form Larger, morecomplex logical functions than can be realized using a single LE 300.Furthermore, because of its reduced size and shorter length, localinterconnect structure 310 has reduced parasitics compared to the globalinterconnection structure. Consequently, local interconnect structure310 generally allows signals to propagate faster than through the globalinterconnect structure.

The memory cell of the present invention may be used in various placesin a PLD. For example, the memory cell may be used to implement andstore the configuration of a look-up table, function generator,programmable interconnect, and other components of the PLD. Using thememory cell of the present invention, a RAM may be constructed that willbe used to implement the look-up tables used in LEs 300. The memory cellof the present invention may also be as a storage bit. This storage bitmay be used, for example, to determine whether an LE is in registered orsequential mode. The storage bit may also be used to configure theoperation of a programmable multiplexer.

FIG. 4 is a diagram showing an example of how the present invention maybe used in the implementation of the programmable interconnectionresources of a PLD, such as shown in FIGS. 2A-D. Memory cell 400 is aprogrammable memory cell of the present invention. Memory cell 400stores a logic state, either a high or low logic state. Typically, alogic high state is represented by a "one" or VDD, while a logic lowstate is represented by a "zero" or ground.

Memory cell 400 is reprogrammable, which means that the cell may beprogrammed many times with new data. Furthermore, in the presentinvention, memory cell 400 is nonvolatile, which means that the storedinformation is retained even when power is removed. For example, aftermemory cell 400 is programmed with a particular logic state, memory cell400 stores this information indefinitely until the cell is reprogrammed.

Memory cell 400 has an output node 405 for outputting its stored state.Output node 405 provides either a logic low output or a logic highoutput, representative of the state stored in memory cell 400. Theoutput voltage levels at output node 405 are typically full-railvoltages, equal to either about VDD (sometimes referred to as VCC) orVSS of the integrated circuit.

Memory cell 400 may be used to implement many types of logic elementsand components. For example, memory cell 400 may be used to implement aflip-flop, register, storage bit, architecture bit, lookup table,programmable interconnect array, RAM, SRAM, ROM, EPROM, EEPROM,content-addressable memory (CAM), PLD, FPGA, PC Card memory card, andother similar memory elements and devices. As shown in FIG. 4, memorycell 400 may be used to implement the programmable interconnectionresources of a PLD.

In FIG. 4, GH 210 and GV 220 conductors cross at intersections 225.Intersections 225 are sometimes referred to as crosspoints. As istypically the case with global programmable interconnect, GH 210 and GV220 lines do not connect at intersections 225 unless they areprogrammably coupled. A pass transistor 410 and programmable memory cell400 facilitate the programmable coupling of GH 210 to GV 220, and viceversa. In particular, depending on the data stored in memory cell 400,GH 210 and GV 220 conductors are programmably connected or programmablydisconnected at intersection 225.

At intersection 225, pass transistor 410 is coupled between GH 210 andGV 220. A gate of pass transistor 410 is coupled to output node 405 ofmemory cell 400. Controlled by way of memory cell 400, pass transistor410 programmably couples GH 210 to GV 220. For example, GV 220 and GH210 may be coupled together by storing a logic high in memory cell 400.Memory cell 400 would output a high level at output node 405, which ispassed to the gate of pass transistor 410. A high at the gate of passtransistor 410 turns pass transistor 410 on, so that GV 220 iselectrically coupled to GH 210. In the alternative, GV 220 and GH 210may be decoupled from another by programming a zero into memory cell400. In this manner, memory cell 400 of the present invention may beused to implement a global programmable interconnect structure for aPLD.

Furthermore, as discussed earlier, the present invention may providefull-rail voltages at the gate of pass transistor 410. In this case,pass transistor 410 will be either substantially on or substantiallyoff. This allows GHs 210 and GVs 220 to conduct signals throughout theintegrated circuit with generally good performance characteristics. Forexample, when pass transistor 410 is fully on (e.g., the gate is atVDD), the amount of resistance of pass transistor 410 is kept at aminimum, thus improving transient performance. Also, VDD at the gate ofpass transistor 410 allows a voltage of about VDD-VT (a thresholdvoltage of pass transistor 410, including body effect) from one GH 210or GV 220 conductor to another. Also, when pass transistor 410 is fullyoff (e.g., the gate is at VSS), no signals will pass (or "leak") from GH210 or GV 220 conductor to another. Further, when pass transistor 410 isfully off, the GV 220 conductors intersecting a particular OH 210conductor will not capacitively load that particular GH 210 conductor.These features improve the performance of the programmable logic deviceintegrated circuit.

Pass transistor 410 may be fabricated using many different processtechnologies, including CMOS, MMOS, PMOS, and bipolar. In a preferredembodiment, pass transistor 410 is an NMOS transistor.

FIG. 5A shows a more detailed block diagram of memory cell 400 of thepresent invention. Memory cell 400 stores a logic state. Memory celloutputs this stored logic state, a logic high or a logic low, ontooutput node 405. For an integrated circuit, the logic Low may berepresented by a first voltage level, and the logic high may berepresented by a second voltage level. The first voltage level is aboutVSS. The second voltage level is generally about the supply voltage forthe integrated circuit, VDD (or VCC). VDD is typically 5 volts and VSSis ground. Furthermore, to facilitate highly integrated semiconductorcircuits, the typical voltage supply of 5 volts has been reduced to avoltage level less than 5 volts, commonly from about 3.3 volts to 3.6volts. In the future, supply voltages will be reduced even further to,for example, 2.5 volts or lower.

Furthermore in an embodiment, the supply voltages for memory cell 400may not the same as the supply voltages for the integrated circuit. Inthis case, the first voltage level may not be about VSS. The secondvoltage level may not be about VDD. For example, VDD may be about 5volts, but this voltage may be reduced to about 3.3 volts for couplingto the internal circuits. In this case, the second voltage level may bea "local" VDD, a VDD which is coupled to memory cell 400 and possiblyother internal circuitry. In a further example, the supply voltage ofthe integrated circuit may be VDD, but the second voltage level formemory cell 400 may be about VDD-VT. In yet another example, VDD may beabout 3.3 volts, and the second voltage level for memory cell 500 may bea higher voltage, about 5 volts.

In FIG. 5A, memory cell 400 includes a first voltage source 505, whichis typically coupled to VSS, and a second voltage source 510, which istypically coupled to VDD. Further, memory cell 400 includes aprogrammable memory element 515 coupled between the second voltagesource 510 and output node 405. Programmable memory element 515 may be anonvolatile memory element, which may be fabricated from many differentmemory technologies. Nonvolatile memory cells retain their storedinformation even when power is removed. For example, programmable memoryelement 515 may be fabricated using one-time programmable devices suchas fuses or antifuses. Furthermore, programmable memory element 515 maybe fabricated using reprogrammable, nonvolatile memory devices such asEPROM, EEPROM, Flash EEPROM, and the like. In a preferred embodiment,programmable memory element 515 is an EEPROM or Flash EEPROM cell.

Further, in memory cell 400, a pull-down device 525 is coupled between afirst voltage source 505 and output node 405. Pull-down device 525 maybe substantially dissimilar to programmable memory element 515.Pull-down device 525 may be formed using many different types ofcomponents, active and passive, including bipolar and MOS transistors,as well as resistors. Pull-down 525 may be programmable ornonprogrammable. In a preferred embodiment, pull-down device 525 is aresistor. On an integrated circuit, a resistor may be fabricated usingmany different techniques and from different devices. For example, aresistor may formed using polysilicon, undoped colysilicon,polysilicide, n-type diffusion, p-type diffusion, n-well diffusion, anda transistor channel, among others. This resistor may be integrated withother devices or formed from the layout of the memory cell by theinterconnect. For example, in a layout, pull-down device 525 may be aserpentine polysilicide interconnect between first voltage source 505and output node 405.

In one embodiment, pull-down device 525 is a resistor of approximatelyone gigaohm resistance. However, this resistor may be larger or smaller.For example, pull-down device 525 is a resistor which may be in therange from about three teraohms to about thirty kiloohms. The resistormay be in the range from about thirty kiloohms to about three megaohms.The resistor may be in the range from about three megaohms to aboutthirty megaohms. The resistor may be from about thirty megaohms to aboutthree hundred megaohms. The resistor may be from about three hundredmegaohms to about one gigaohms. The resistor may be from about onegigaohm to about three gigaohms. The resistor may be from about threegigaohms to about ten gigaohms. The resistor may be from about tengigaohms to about thirty gigaohms. The resistor may be from about thirtygigaohms to about three hundred gigaohms. The resistor may be from aboutthree hundred gigaohms to about three teraohms. The resistor may begreater than three teraohms. As specific examples, a resistor of aboutfifty megaohms may be used. A resistor of about one hundred megaohms maybe used. A resistor of about two hundred megaohms may be used. Aresistor of about five hundred megaohms may be used.

The programmable memory element 515 will typically have a resistance onthe order of ten kiloohms to one megaohm in the erased state, and aresistance of one to one hundred teraohms in the programmed state. Theload resistor should have a resistance intermediate between these tworanges. The precise values of programmed and erased resistance of memoryelement 515, and hence the optimum load resistance value, will besomewhat technology dependent.

There are currently two preferred methods of manufacture for loadresistors to achieve high resistance as used in the present invention.The first is to use undoped, lightly ion-implanted polycrystallinesilicon as the load resistor. Resistance values for such resistors canbe controlled over a very broad range of from less than one hundredkiloohms to more than one teraohm. This technique has the advantage ofrelatively simple processing. This approach is practical as long as thetotal number of memory cells is less than several million. For memorycell counts in the range of ten million or more, standby currentrequirements may dictate a need for higher load resistance. With currenttechnology, a commonly used approach is to use a p-channel thin-filmtransistor (TFT) as the load element in this case.

FIG. 5B shows an example of an embodiment of the present invention usinga TFT 525. Such a device is switched between an off state withresistance on the order of fifty teraohms and an on state withresistance on the order of a few hundred megaohms. By coupling a gate ofsuch a device to a gate of the programmable memory element 515, forexample, by having them share the same Floating gate 529 (as shown inFIG. 5B), the TFT can be switched to its on state when memory element515 is programmed, and to its off state when memory element 515 iserased. In this way, very low standby currents can be achieved whilestill satisfying the resistance ratio requirements needed to deliverfull, or nearly full, VDD and VSS output levels from the cell.

In operation, memory cell 400 stores and outputs a logic low or logichigh. Data is stored in memory cell 400 by programming programmablememory element 515. Programmable memory element 515 has two states,programmed or erased (not programmed). Depending on the memorytechnology used, the terms "programmed" and "erased" may have differentmeanings. In this specification, The IEEE convention for these terms isused. Specifically, "programmed" refers to placing a memory element intoan "off" or nonconducting state, and "erased" refers to placing a memoryelement into an "on" or conducting state.

Memory cell 400 produces an output representative of the data stored inprogrammable memory element 515 at output node 405. When programmablememory element 515 is programmed, output node 405 will be decoupled fromsecond voltage source 510. A standby pull-down current, or firstpull-down current, to first voltage source 505 through pull-down device525 will pull output node 405 to about the first voltage level,representing a logic low. Except for leakage currents, the firstpull-down current may be about zero microamps. In this state, memorycell 400 consumes no static power.

When programmable memory element 515 is erased, output node 405 iscoupled through programmable memory element 515 to second voltage source510. A pull-down current, or second pull-down current, to first voltagesource 505 through pull-down device 525 will be drained throughprogrammable memory element 515 from second voltage source 510, or VDD.Output node 405 will be at about the second voltage level, a logic high.The magnitude of the pull-down current depends on the size of theresistor and the levels of the first voltage source 505 and secondvoltage source 510; this current may be calculated by Kirchhoff's laws.For example, pull-down device 525 may be about three gigaohms, and thepull-down current may be less than about one nanoamp.

The pull-down current of the memory cell 400 may be from about onepicoamp to about one hundred microamps. The pull-down current may befrom about one hundred microamps to about one microamp. The pull-downcurrent may be from about one microamp to about one hundred nanoamps.The pull-down current may be from about one hundred nanoamds to aboutten nanoamps. The pull-down current may be from about ten nanoamps toabout three nanoamps. The pull-down current may be from about threenanoamps to about one nanoamps. The pull-down current may be From aboutone nanoamp to about three hundred picoamps. The pull-down current maybe from about three hundred picoamps to about one hundred picoamps. Thepull down current may be from about one hundred picoamps to about tenpicoamps. The pull-down current may be from about ten picoamps to aboutone picoamp. The pull-down current may be less than about one picoamp.As specific examples, the pull-down current may be about two microamps.The pull-down current may be a less than about half a nanoamp, or evenless.

This pull-down current will be the current consumption of memory cell400. Since memory cell 400 consumes a relatively small amount ofcurrent, large arrays of memory cells 400 may be constructed, and thecombined power consumption of the entire array of memory cells remainsreasonable. For example, if a memory cell consumes a maximum of aboutone nanoamp, one million memory cells will consume at most about onemilliamp. The exact power consumption will depend on whether the memorycells are programmed or erased. Therefore, in a programmable logicdevice, the power consumption of an array of memory cells 400 of thepresent invention will be pattern dependent.

Memory cell 400 of the present invention may provide approximatelyfull-rail voltage output at output node 405. More specifically, whenprogrammable memory element 515 is programmed, output node 405 will beat approximately the first voltage level since the off resistance of theprogrammable memory element 515 will be significantly more than theresistance of pull-down device 525. When programmable memory element 515is erased, output node 405 will be at approximately the second voltagelevel since the on resistance of the programmable memory element 515will be significantly less than the resistance of pull-down device 525.

Memory cell 400 of the present invention may also include features toallow the programming and erasure of programmable memory element 515.More specifically, EPROM EEPROM, Flash EEPROM cells have floating gatesto provide for nonvolatility and reprogrammability. To program and erasethese cells, high voltages may be used to transfer charge to and removecharge from the floating gates through the silicon oxide by variousphysical mechanisms such as avalanche injection, channel injection,quantum-mechanical tunneling, hot electrons, and other phenomena.

A high voltage (VPP) used to program the memory cells may be somewhatdifferent from a high voltage (VEE) used to erase the memory cells. Themagnitude of VPP and VEE depends on the physical characteristics of thememory cell including such considerations as the thickness andcomposition of the dielectric between the substrate and the floatinggate. Typically, VPP and VEE voltages are in the range of approximately11 volts to 15 volts. However, as process technology continues toimprove, it will become possible to fabricate thinner and betterdielectrics. Consequently, the high voltages needed to program and erasethe memory cells may decrease in magnitude.

For EEPROM and Flash EEPROM memory cells, to erase the cells, charge iselectrically removed from the floating gate using high voltages andquantum-mechanical tunneling. For EPROM memory cells, charge is removedfrom the floating gates by exposing the cells to radiation, such asultraviolet light; exposing EPROM memory cells to ultraviolet lightallows the charge in the floating gate to escape. After erasure, EPROM,EEPROM, and Flash EEPROM memory cells may be reprogrammed. EPROM andFlash EEPROM memory cells are generally programmed using hot electrons,while EEPROM cells are programmed using quantum-mechanical tunneling. Insome cases, Flash EEPROM memory cells are programmed usingquantum-mechanical tunneling. EPROM, EEPROM, and Flash memory cells maybe programmed, erased, and reprogrammed many times.

When EPROM, EEPROM, and Flash EEPROM memory cells are programmed,negative charge (e.g., electrons) is placed on the floating gate and amemory cell is placed in a high voltage threshold (VT) state (VTP). Inthe programmed or high VT state, a reasonable voltage on a gate of thememory cell will not turn the memory cell on. In an embodiment, areasonable voltage is a voltage between the supplies of the integratedcircuit, VDD and VSS. Voltages in this range are readily available, andmay be generated without relatively complex circuitry. However, voltageshigher than VDD may also be used as the reasonable voltages. Voltagesabove VDD may be generated, for example, by using charge pumps.Furthermore, in the case when a voltage above second voltage source 510is placed on the gate, the memory cell may turn on. For example, forEPROM, EEPROM, and Flash EEPROM memory cells, the typical thresholdvoltage for a memory cell in a high VT state is approximately 5 volts.However, the high VT state may also be above 5 volts. For example, forEPROM memory cells, the high VT state may be about 7 volts. Whenprogrammed using an "assist" technique (described below), the high VTstate for EEPROM memory cells may be somewhat higher than 5 volts.

In contrast, when erased, negative charge is removed from the floatinggate and the memory cell is placed in a low VT state (VTE). In theerased or low VT state, a reasonable voltage on a gate of the memorycell will turn the memory cell on. For EEPROM and Flash EEPROM memorycells, the typical threshold voltage for a memory cell in a low VT stateranges from approximately -2 volts to -4 volts. For EPROM memory cells,the typical threshold voltage in a low VT state is zero volts orslightly above zero volts.

When EEPROM or Flash EEPROM cells are erased using a "smart" algorithm,a specific low VT voltage level state may be achieved. For example, theVTE may be about -1 volts. In another example, the VTE may be zero voltsor slightly above zero volts. A smart algorithm removes charges from thefloating gate using an iterative technique, removing small amounts ofcharge until a desired VTE is achieved. A smart algorithm may be used toprevent a negative VTE, especially important in some applications wherea negative VTE is undesirable such as for high-density, high-capacityFlash EEPROM memories. In other applications such as programmable logicdevices, a negative low VT state may be allowable because, among otherreasons, the memory cell may be accessed using a read transistor inseries with the memory cell. The present invention does not require theuse of a smart algorithm since memory cell 400 operates and functionsproperly when programmable memory element 515 has a negative VTE.

Also, when an array of EPROM or Flash EEPROM memory cells isinitialized, the cells are erased to a low VT state. For example, anEPROM array is initialized when exposed to ultraviolet light. FlashEEPROM memory cells are initialized when bulk erased. However, when anarray of EEPROM memory cells is initialized, the cells typically may beprogrammed to a high VT state. The initiation state of a memory array ischosen based on many considerations including the fabrication of thememory cell devices themselves and the intended use for the memorycells.

A control gate 520 activates programmable memory element 515. Duringnormal operation, control gate 520 is coupled to a voltage level whichactivates and enables operation of programmable memory element 515.Until activated, programmable memory element 515 may decouple secondvoltage source 510 from output node 405 during normal operation. Toactivate programmable memory element 515, control gate 520 is coupled toa third voltage level which is greater than VTE and less than the VTP.For example, for a typical EEPROM cell, control gate 520 should aboveabout -2 volts (VTE) and less than about 5 volts (VTP). In a preferredembodiment of the present invention, in which the programmable memoryelement 515 is the pull-up device to the VDD power supply, the thirdvoltage level is chosen to provide the greatest amount of programmingand erase margin for extreme cases of operation. For the programmedstate, one extreme case is the typical one of zero source bias. For theerased state, however, the extreme case is with source bias equal toVDD. In this case, the programmable memory element 515 should remainconducting. For example, if VTE is about -4 volts (without source bias)and VTP is about 5 volts, and if VDD is 3.3 volts, the control gate maybe set at about VDD to provide about 1.5 volts or more of margin for thetwo extreme cases discussed. As a further example, for an EPROM memorycell, where VTE equals about zero volts and VTP equals about 7 volts,control gate 520 may be tied to a higher voltage level than VDD. Anactivation voltage above VDD may be used because for example, this maybe required by the memory element, or a higher voltage level is readilyavailable. Control gate 520 is also used during the programming ofprogrammable memory cell 515, which is described in more detail below.

FIG. 5C shows an embodiment of the present invention where programmablememory element 515 is coupled between output node 405 and first voltagesource 505. A pull-up device 525B is coupled between output node 405 andsecond voltage source 510. The embodiment shown in this figure operatesin analogous fashion to that described for FIG. 5A. Programmable memoryelement 515 would be used to couple output node 405 to a logic lowlevel. Pull-up device 525B would be used to couple output node 405 to alogic high level.

The embodiment in FIG. 5A has some advantages compared to the cell inFIG. 5C. In particular, during the operation of an electronic circuit,switching noise may be coupled into node 405 from the operation ofcircuitry coupled to node 405. This noise tends to be of a highermagnitude when output node 405 is at a logic high, rather than at alogic low. For example, as shown in FIG. 4, node 405 may be coupled to apass transistor 410, which is between GH and GV conductors. When passtransistor 410 is in the on state, switching noise from signals on theGH and GV conductors is coupled to node 405. The voltages coupled tonode 405 from switching noise may be in the positive or negativedirection. As an example, node 405 may be boosted to a higher voltageduring low-to-high transitions on the GH and GV conductors. It isundesirable to propagate this noise to other elements in the circuitsince this may be a source of logical errors. These boosted voltages mayalso damage or decrease the longevity of the memory cell.

Under the above conditions, the memory cell of FIG. 5A will tend to holdnode 405 to a more stable voltage than the embodiment in FIG. 5C. Thisis because any voltage coupled into node 405, when it is a logic high,can be discharged (relatively quickly) through the relatively lowimpedance of memory element 515 for FIG. 5A. Compared to theconfiguration of FIG. 5C, boosted voltages at node 405 may not bedischarged as easily because of the relatively large impedance ofresistor 525B.

On the other hand, for the embodiment of FIG. 5C, when programmablememory element 515 is in the erased state, there is a reduced electricfield between the channel and the substrate of programmable memoryelement 515. Specifically, when a floating gate memory cell (such as anEEPROM or Flash EEPROM) is used, the floating gate is not subjected to achannel-to-substrate bias (referred to as floating gate disturbphenomena). The channel of programmable memory element 515 in theembodiment of FIG. 5C is at zero volts or cutoff. A channel-to-substratebias generates electron-hole pairs. These electrons may be attracted tothe floating gate, which may disturb the memory cell and reduce thelifetime of the device. Therefore, with respect to this phenomenon, theembodiment of FIG. 5C of the present invention would have excellent dataretention and reliability characteristics, even when VDD is 5 volts. Inpractice, the choice between the embodiment of FIG. 5A versus FIG. 5Cmay be made based on careful consideration of the specific technologyavailable and the circuit noise environment.

FIG. 6 shows a diagram of a specific embodiment of a memory cell 400 ofthe present invention. In this embodiment, memory cell 400 isimplemented using an EEPROM memory cell 600. An array of memory cells400 may be constructed mirroring and repeating the memory cell inhorizontal and vertical directions. In FIG. 6, the programmable memoryelement is implemented using a single- or double-polysilicon EEPROM cell610. Further, memory cell 400 of FIG. 6 includes additional transistorsand signal lines to provide for the programming of EEPROM cell 610.

Memory cell 400 includes the following devices: EEPROM cell 610,pull-down device 525, and a select transistor 620. Furthermore, memorycell 400 has a tunnel dielectric 630 for transferring electrons from atunnel diode 640 to a floating gate 635 of EEPROM cell 610. EEPROM cell610 is coupled between second voltage source 510 and output node 405.EEPROM cell 610 has a control gate 520. Pull-down device 525 is coupledbetween first voltage source 505 and output node 405. In a preferredembodiment, pull-down device 525 is a resistor, as discussed Previously.Select transistor 620 is coupled between tunnel diode 640 and an erasenode 650. Select transistor has a select gate 660.

Memory cell 400 of FIG. 6 operates substantially similarly to memorycell 400 of FIG. 5A. EEPROM cell 610 is configured to store the data ofmemory cell 400. Data stored in memory cell 400 is output at output node405. When initialized, an array of EEPROM cells is typically bulkprogrammed to a high VT state. A typical VTP For an EEPROM cell is about5 volts to 6 volts. In a high VT state, an EEPROM cell decouples itsdrain terminal from its source terminal. When erased to a low VT state,an EEPROM cell couples its drain terminal to its source terminal. Atypical VTE for EEPROM cells is about -3 volts.

By appropriately configuring EEPROM cell 610, memory cell 400 will storea logic high or logic low. To store a logic low, EEPROM cell 610 remainsprogrammed. Then, output node 405 will be pulled to a logic low, at thefirst voltage level, by a first pull-down current to first voltagesource 505 through pull-down device 525. To store a logic high, EEPROMcell 610 is erased. A pull-down current to first voltage source 505through pull-down device 525 will be drained through EEPROM cell 610from second source 510. Output node 405 will approximately equal thesecond voltage level or VDD, a logic high.

Control gate 520 activates the EEPROM cell 610. As discussed, duringnormal operation, control gate 520 is set at a voltage between the VTEand VTP of EEPROM cell 610 to maximize the amount of programming anderase margin. In one embodiment, this voltage is approximately VDD(e.g., 3.3 volts).

Control gate 520 is also used during the programming of EEPROM cell 610.To initialize (or program) EEPROM cell 610, control gate 520 is set toVPP. Erase node 650 is grounded. Select gate 660 is set at a voltage toturn on select transistor 620. Select gate 660 is typically set to avoltage of about 2 volts, although any voltage in the range from 2 voltsto VPP will also turn on select transistor 620. Second voltage source510 and first voltage source 505 are grounded. Under these conditions,electrons will tunnel from tunnel diode 640 through tunnel dielectric630 into floating gate 635 of EEPROM cell 610. Floating gate 635 becomesnegatively charged so that EEPROM cell 610 is programmed to the high VTstate. This may be done globally to program all bits in an array ofEEPROM cells 610.

By using an assist programming technique, VTP may be boosted to a highervalue. For assist programming, second voltage source 510 is set to anassist voltage of about ODD. First voltage source 505 is allowed tofloat (or set at VDD). Since control gate 520 is at VPP, output node 405is also at about VDD. First voltage source 505 will float to about VDDthrough pull-down device 525. Under these conditions, there isadditional voltage coupling to floating gate 635 through the channel ofEEPROM cell 610 that boosts the initial voltage of the floating gate.When the initial voltage of the floating gate is higher, the resultingVTP voltage will be higher. Consequently, more electrons become trappedin floating gate 635 than under the programming conditions withoutassist. Hence, floating gate 635 is programmed to a higher VTP level.Furthermore, the assist voltage may be above VDD to increase the voltagecoupling to floating gate 635. For example, the assist voltage may beabout VPP.

EEPROM cells are typically programmed using quantum-mechanicaltunneling. By using quantum-mechanical tunneling, substantially lesscurrent is needed to program EEPROM cells than for example, Flash EEPROMcells, which are typically programmed using hot electrons. EEPROM cellsare programmed primarily due to the magnitude of the high voltagesacross very thin dielectrics (typically seventy-five angstroms to ninetyangstroms of oxide). Therefore, since high currents are not necessary,the use of EEPROM cells generally provides integrated circuits which maybe configured while resident on a system board--sometimes referred to asin-system programming (ISP)--where large currents are often unavailable.

Quantum-mechanical tunneling is also the mechanism used to erase EEPROMcell 610. EEPROM cells are selectively erased. For example, to eraseEEPROM cell 610, select gate 660 is set to VEE+VT (a threshold voltageof select transistor 620). Erase node 650 is coupled to VEE. Controlgate 520, first voltage source 505, and second voltage source 510 aregrounded. VEE is passed through select transistor 620 to tunnel diode640. Electrons from floating gate 635 of EEPROM cell 610 are attractedthrough tunnel dielectric 630 into tunnel diode 640. This removeselectrons from the floating gate 635 of EEPROM cell 610. Thus, EEPROM610 is erased to a low VT state. For an array of EEPROM cells 610, thisoperation is repeated in a row-by-row or column-by-column basis untilthe desired pattern is imprinted into the array.

When erasing EEPROM cells 610, a negative assist programming techniquemay be used by placing a negative assist voltage on control gate 520.For example, a voltage of -3 volts may be placed on control gate 520.This would decrease VTE, improving the erase margin of EEPROM cell 610.

An array of EEPROM cells may be monitored to verify the program anderase states of the bits. Several methods for this are possible. In onemethod, to verify the program margin, the erase nodes 650 of the arrayare grounded. Second voltage sources 510 are set to about 2 volts. Thevoltage at a control gate 520, corresponding to the row of cells beingverified, is swept until an EEPROM cell 610 begins conducting and thecorresponding first voltage source 505 line begins conducting. Thisprocedure allows the verification of one row of EEPROM cells 610 at atime. All of the bits in that row are verified through separate firstvoltage source 505 lines.

FIG. 7 shows memory cell 400 of the present invention implemented usingtwo-transistor double-polysilicon Flash EEPROM memory cells. This cellmay be repeated to form an array of memory cells. In this embodiment,memory cell 400 includes a Flash EEPROM read cell 710 and a Flash EEPROMprogram cell 715. Flash EEPROM read cell 710 shares a floating gate 735with Flash EEPROM program cell 715. Flash EEPROM read cell 710 and FlashEEPROM program cell 715 are configured to store data in memory cell 400.

Flash EEPROM read cell 710 is coupled between second voltage source 510and output node 405. Flash EEPROM program cell 715 is coupled betweenprogram node 740 and erase node 745. A pull-down device 525 is coupledbetween first voltage source 505 and output node 405. Data stored inmemory cell 400 is read or output from output node 405. Depending on theprogrammed state of Flash EEPROM read cell 710, output node 405 willprovide a logic high or logic low output. Flash EEPROM program cell 715facilitates the configuration of EEPROM read cell 710.

A control gate 520 is coupled to a gate of Flash EEPROM read cell 710and a gate of Flash EEPROM program cell 715. A control gate 520 isanalogous to control gate 520 of FIG. 5A. Control gate 520 may be usedto activate Flash EEPROM read cell 710. For normal operation of memorycell 400, control gate 520 is set at a voltage so that approximatelyfull-rail voltage output is provided at output node 405. Morespecifically, during normal operation, control gate 520 is set at avoltage between the VTE and VTP of Flash EEPROM read cell 710 tomaximize the amount of programming and erase margin. In one embodiment,this voltage is approximately VDD (e.g., 3.3 volts).

By appropriately configuring Flash EEPROM read cell 710 in either aprogrammed or erased state, memory cell 400 may store a logic low in afirst state and a logic high in a second state. Flash EEPROM memorycells are initialized to a low VT state. To store a logic high, FlashEEPROM read cell 710 remains erased in a low VT state. A pull-downcurrent (second pull-down current) through pull-down device 525 will bedrained through Flash EEPROM read cell 710 from second voltage source510. Output node 405 will be at a voltage approximately equal to secondvoltage source 510, representing a logic high. To store a logic low,Flash EEPROM read cell 710 is programmed to a high VT state. A standbypull-down current (first pull-down current) to first voltage source 505through pull-down device 525 pulls output node 405 to a logic low. Then,output node 405 will approximately equal the level of the first voltagesource 505, which represents a logic low.

Flash EEPROM program transistor 715 is used to program and erase FlashEEPROM read cell 710. To initialize (erase) Flash EEPROM cells 710 and715 to a low VT state, VEE is placed at erase node 745. Control gate 520is held at a low voltage, grounded or negative. First voltage source505, second voltage source 510, and program node 740 are grounded. VEEis coupled to a tunnel dielectric (TD) source 765, which is coupled toFlash EEPROM program cell 715. TD source 765 is a diffusion region ofmemory cell 400. For example, TD source 765 may be comprised ofn+diffusion. TD source 765 may also be separate diffusion regionscoupled together via a conductor. Typically, one or more tunneldielectrics are positioned above TD source 765. From TD source 765, viathe appropriate tunnel dielectric, electrons may pass to and from thefloating gate of Flash EEPROM program cell 715, and consequently throughthe shared floating gate to the floating gate of Flash EEPROM read cell710. A high voltage VEE at TD source 765 attracts electrons out of thefloating gates of Flash EEPROM cells 710 and 715. Flash EEPROM cells 710and 715 are erased using quantum-mechanical tunneling. This places FlashEEPROM cells 710 and 715 in a low VT state. The VTE of Flash EEPROMcells 710 and 715 is approximately -3 volts. The time to erase FlashEEPROM cells 710 and 715 is typically about one hundred milliseconds.However, the erase time may be less than one hundred milliseconds, andmay also be very much greater than one hundred milliseconds.

To program memory cell 400 to a logic low, Flash EEPROM cells 710 and715 are programmed to a high VT state. To program Flash EEPROM cells 710and 715, VPP is placed on control gate 520. A VPD voltage is placed onprogram node 740. VPD is selected to optimize the efficient programmingof Flash EEPROM program cell 715. These considerations include ensuringthat the Flash EEPROM cells 710 and 715 are programmed well, minimizingany destructive effects on the cell, and observing the current densityconstraints of the power conductors. In one embodiment, VPD is about 6volts. Erase node 745, first voltage source 505, and second voltagesource 510 are grounded.

Under these conditions, a significant current flows from program node740 through Flash EEPROM program cell 715 to erase node 745. Forexample, this current may be about five hundred microamps. This currentis typically in the range from about one hundred microamps to onemilliamp. This current generates hot electrons, some of which jump adielectric barrier and become trapped in the shared floating gate ofFlash EEPROM cells 710 and 715. This floating gate becomes negativelycharged. Flash EEPROM cells 710 and 715 are programmed to a high VTstate, via shared floating gate 735. VTP is typically about 5 volts to 6volts.

Furthermore, VTP may be boosted to a higher value by using an "assist"programming technique. In assist programming, when programming FlashEEPROM cells 710 and 715, second voltage source 510 is set at about VDDor higher. Since control gate 520 is VPP, output node 405 is also atabout VDD or higher. Under these conditions, more electrons becometrapped in the floating gate than under the programming conditionsdescribed above. Hence, the floating gate is programmed to a higher VTP.

The shared floating gate configuration, as shown in FIG. 7, has severaladvantages related to layout and processing considerations. The layoutof the memory cell 400 of FIG. 7 minimizes the sizes of the topographysteps due to the semiconductor processing, so that the metal lines andother conductors which run across an array of memory cells 400 will notbecome too thin. More specifically, as the steps of an integratedcircuit become steeper, the metal lines that traverse over and coverthese steps become thinner at the corners of these steps. If the metallines become too thin, this may lead to breaks or fractures in thelines. The layout of the shared floating gate configuration shown inFIG. 7 tends to have a flatter topography, which minimizes the stepsizes. This feature of the present invention heaps prevent the problemsof breaks and fractures in the metal lines as well as other processlayers.

The present invention shares floating gate 735 between Flash EEPROM readcell 710 and Flash EEPROM program cell 715. Sharing floating gate 735enables the optimization of the programming and read characteristics ofthe Flash EEPROM program and read cell to achieve longer devicelifespans, improve data retention, and increase performance. A reasonfor this is that Flash EEPROM read cell 710 and Flash EEPROM programcell 715 can, for example, be doped, controlled, and processed somewhatindependently from each other. For example, Flash EEPROM program cell715 can be optimized to improve the ease of programming floating gate735; and, Flash EEPROM read cell 710 may be optimized to improve theprogramming margin and charge retention characteristics of the floatinggate during normal operation, thereby increasing memory cell longevity.

In other embodiments of the present invention, another TD source may becoupled to Flash EEPROM read cell 710, similarly to Flash EEPROM programcell 715. This additional TD source and associated tunnel dielectricswill facilitate the convenient and efficient transfer of electrons toand from the shared floating gate 735. However, when only one TD source765 is used, as shown in FIG. 7, memory cell 400 can still be programmedefficiently, and the processing and fabrication of memory cell 400 issomewhat simplified. Overall, the embodiment of the present inventionshown in FIG. 7 produces memory cells 400 with higher read current,better programming characteristics, and enhanced device longevitycompared to Flash EEPROM cells with only one floating gate transistor.

Programming and erase for an array of memory cells is generallyanalogous to the techniques described above. Further, when programmingan array of memory cells, there are many techniques for ensuring onlythe desired EEPROM cells are programmed, while others are leftundisturbed. For example, if the EEPROM cells have a depletion VT, thecells not selected for Programming may be deselected by using a negativecontrol gate voltage. A further technique For deselecting particularEEPROM cells may involve back biasing the wells of the unselected cells.

FIG. 8 shows memory cell 400 of the present invention implemented using"one-and-a-half"-transistor single- or double-polysilicon Flash EEPROMmemory cells. This cell may be repeated to form an array of memorycells. The electrical characteristics of the Flash EEPROM memory cellsin this configuration are similar to those discussed above. Thisembodiment of memory cell 400 is very compact and includes the followingdevices: a Flash EEPROM cell 810, a tunnel diode 860, and a tunneldielectric 865. Flash EEPROM cell 810 has a floating gate 870.

Flash EEPROM cell 810 is configured to store data in memory cell 400.Flash EEPROM cell 810 is coupled between output node 405 and secondvoltage source 510. A pull-down device 525 is coupled between firstvoltage source 505 and output node 405. In this embodiment, pull-downdevice 525 is a resister. Data stored in memory cell 400 is read oroutput from output node 405. Depending on the state of Flash EEPROM cell810, output node 405 will represent a logic high or logic low.

Tunnel diode 860 facilitates the configuration of Flash EEPROM cell 810.Tunnel diode 860 is coupled to erase node 845. Tunnel diode 860 iscoupled by tunnel dielectric 865 to floating gate 870 of Flash EEPROMcell 810. Control gate 520 is coupled to a gate of Flash EEPROM cell810. Control gate 520 is used to activate Flash EEPROM cell 810. Fornormal operation of memory cell 400, control gate 520 is set at avoltage so that approximately full-rail voltage output is provided atoutput node 405. More specifically, as described earlier, during normaloperation, control gate 520 is set at a voltage between the VTE and VTPof Flash EEPROM cell 810 to maximize the amount of programming and erasemargin. In one embodiment, this voltage is approximately VDD (e.g., 3.3volts).

By appropriately configuring Flash EEPROM cell 810 in either programmedor erased states, memory cell 400 may store a logic low in a first stateand a logic high in a second state. Flash EEPROM memory cells areinitialized to a high VT state. In the first state, to store a logiclow, Flash EEPROM cell 810 remains programmed in a high VT state. Astandby pull-down current (first pull-down current) to first voltagesource 505 through pull-down device 525 will pull output node 405 to alogic low. Output node 405 will approximately equal the level of thefirst voltage source 505. In the second state, to store a logic high,Flash EEPROM cell 810 is erased to a low VT state. A pull-down current(second pull-down current) through pull-down device 525 will be drainedthrough Flash EEPROM cell 810 from the second voltage source 510. Then,output node 405 will approximately equal the voltage level of secondvoltage source 510, which represents a logic high.

In this embodiment, Flash EEPROM cell 810 may be programmed and erasedusing quantum-mechanical tunneling. This may be referred to as full E²operation of the memory cell. The following example shows how thisconfiguration of Flash EEPROM cells may be programmed and erased usingfull E² operation. In this case, the Flash EEPROM cells are initialized(or bulk programmed) to the high VT state. Control gate 520 is coupledto VPP. First voltage source 505, second voltage source 510, and erasenode 845 are grounded. Electrons tunnel from tunnel diode 860, throughtunnel dielectric 865, to floating gate 870. As a result, Flash EEPROMcell 810 is programmed to the high VT state. An assist technique mayalso be used to achieve a high VTP by placing VDD or a higher voltage atfirst voltage source 505 or second voltage source 510.

In full E² operation, the Flash EEPROM cells are selectively erased to alow VT state. For example, to erase Flash EEPROM cell 810, control gate520 is grounded or placed at a negative voltage (for negative assistprogramming as discussed above). Second voltage source 510 is grounded.First voltage source 505 may be floating or grounded. Erase node 845 iscoupled to VEE. Electrons will tunnel from the floating gate of FlashEEPROM cell 810 through tunnel dielectric 865 to tunnel diode 860. FlashEEPROM cell 810 is erased to a low VT state.

In full E² operation of the memory cell of FIG. 8, the Flash EEPROM cellmay also be initialized i.e., bulk erased) to the low VT state. In thiscase, control gate 520 will be grounded or placed at a negative voltage.Second voltage source 510 is grounded. Erase node 845 is coupled to VEE.The Flash EEPROM cells are selectively programmed to the high VT state.For example, to program Flash EPROM cell 810, control gate 520 iscoupled to VPP. First voltage source 505, second voltage source 510, anderase node 845 are grounded. Electrons tunnel from tunnel diode 860,through tunnel dielectric 865, to floating gate 870. As a result, FlashEEPROM cell 810 is programmed to the high VT state. An assist techniquemay also be used to achieve a high VTP by placing VDD or a highervoltage at first voltage source 505 or second voltage source 510.

To avoid programming or erasing unselected neighboring cells,intermediate, or half-select, voltages may be placed on control gate 520and erase nodes 845 of the neighboring cells. These intermediatevoltages are sufficiently low to avoid (or prevent) either reprogrammingof previously erased cells or erasing of programmed cells which areintended to remain programmed, or vice versa for the blanket initialprogramming embodiment.

The foregoing description of preferred embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform described, and many modifications and variations are possible inlight of the teaching above. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

What is claimed is:
 1. A memory comprising:a tunnel diode providing asource of electrons; an erase node coupled to provide voltages to saidtunnel diode; a pull-down device coupled between an output node and afirst conductor, wherein the pull-down device reduces an output voltageat the output node; a programmable memory element, coupled between saidoutput node and a second conductor; and a tunnel dielectric, coupledbetween said tunnel diode and a gate of said programmable memoryelement, wherein said tunnel dielectric passes electrons between saidtunnel diode and said gate of said programmable memory element.
 2. Thememory of claim 1 wherein said second conductor is at a voltage levelabove said first conductor during normal operation.
 3. The memory ofclaim 1 wherein said pull-down device is a thin film transistor having agate coupled to said gate of said programmable memory element.
 4. Thememory of claim 1 wherein to program said programmable memory element, aprogram voltage is coupled to a control gate of said programmable memoryelement, an assist voltage is placed on said first conductor and saidsecond conductor, and said erase node is grounded.
 5. The memory ofclaim 4 wherein said assist voltage is at a level of a VDD voltage orgreater.
 6. The memory of claim 1 wherein to erase said programmablememory element, a control gate of said programmable memory element isset at ground or less and said erase node is coupled to an erasevoltage.
 7. The memory of claim 1 wherein an intermediate voltage isplaced on control gates of unselected neighboring memory cells.
 8. Thememory of claim 1 wherein said pull-down device is a thin filmtransistor sharing a floating gate with said programmable memoryelement.
 9. The memory of claim 1 wherein the programmable memoryelement is a floating gate device.
 10. The memory of claim 1 wherein theprogrammable memory element is an EEPROM, Flash, or EPROM device. 11.The memory of claim 1 wherein in a layout of the memory, the first andsecond conductors are positioned parallel to each other.
 12. The memoryof claim 1 wherein a logic high level at the output node is staticallyheld by the programmable memory element.
 13. A programmable logic devicecomprising a memory as recited in claim
 1. 14. The memory of claim 1wherein the pull-down device provides a first resistance value, betweena second resistance value of the programmable memory element in anerased state and a third resistance value of the programmable memoryelement in a programmed state.
 15. The memory of claim 1 wherein thepull-down device provides a resistance of more than about one megaohm.16. The memory of claim 1 wherein the pull-down device provides aresistance of more than about one hundred megaohms.
 17. The memory ofclaim 1 wherein the pull-down device provides a resistance of more thanabout one gigaohm.
 18. The memory of claim 1 wherein the pull-downdevice provides a resistance of more than about twenty gigaohms.
 19. Thememory of claim 1 wherein the pull-down device provides a resistance ofabout one teraohm or more.
 20. A memory comprising:a tunnel diodeproviding a source of electrons; an erase node coupled to providevoltages to said tunnel diode; a pull-down device coupled between anoutput node and a first conductor; a programmable memory element,coupled between said output node and a second conductor; and a tunneldielectric, coupled between said tunnel diode and a gate of saidprogrammable memory element, wherein said tunnel dielectric passeselectrons between said tunnel diode and said gate of said programmablememory element, wherein a pull-down current through the pull-down deviceprovides a static logic low output at the output node.
 21. A memorycomprising:a tunnel diode providing a source of electrons; an erase nodecoupled to provide voltages to said tunnel diode; a pull-down devicecoupled between an output node and a first conductor; a programmablememory element, coupled between said output node and a second conductor;and a tunnel dielectric coupled between said tunnel diode and a gate ofsaid programmable memory element, wherein said tunnel dielectric passeselectrons between said tunnel diode and said gate of said programmablememory element, wherein a pull-down current through the pull-down devicereduces a voltage level at the output node.
 22. A memory comprising:atunnel diode providing a source of electrons; an erase node coupled toprovide voltages to said tunnel diode; a pull-down device coupledbetween an output node and a first conductor; a programmable memoryelement, coupled between said output node and a second conductor; and atunnel dielectric, coupled between said tunnel diode and a gate of saidprogrammable memory element, wherein said tunnel dielectric passeselectrons between said tunnel diode and said gate of said programmablememory element, wherein the pull-down device discharges current to thefirst conductor.